Department of Electrical and Computer Engineering, University of Cyprus

FPGA Laboratory Assignment 2

Due Date: 18/10/2018

Aim The purpose of this lab is to show you what are the basic steps you need to take in order to prepare

your design for FPGA download, and verify its operation on the FPGA.

Objectives • Verify that the FPGA board is working!

• Download and verify designs on the FPGA.

Equipment Xilinx Vivado

ISE Simulator (Isim)

Your FPGA Development Board

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Introduction The programmable logic boards used for ECE 408 are Xilinx Zedboards and Nexys 4 (DDR and

Video) development systems. The centerpiece of the boards are Xilinx Atrix and Zynq (field-

programmable gate arrays), which can be programmed via a USB cable or compact flash card. The

board also features I/O ports such as PS/2, serial, Ethernet, stereo audio and video ports, user

buttons, switches and LEDS, and expansion ports for connecting to other boards

Figure 1. The Xilinx Nexys Board

Caution!

The boards contain many exposed components that are sensitive to static electricity. Before

touching the boards, try to remember to discharge any static electricity you may have built up

by touching a grounded piece of metal (i.e. part of the desk). Especially remember to do this

after you have been walking around the room on a carpeted floor (please keep your shoes on).

Preliminaries

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The software for programming the FPGA is the Xilinx Vivado Design Suite introduced in the

previous lab. If you still have questions, then you can follow the following guide:

https://reference.digilentinc.com/vivado/installing-vivado/start

https://reference.digilentinc.com/vivado/getting_started/start

Creating a Project in VIVADO for the Nexys Board: For this lab, we will first follow the tutorial:

https://reference.digilentinc.com/learn/programmable-logic/tutorials/nexys-4-ddr-programming-

guide/start

You can download the files from the website above.

Coding your switch module In order to be able to access the I/O pins and subsequently the board’s features, you need to follow:

https://reference.digilentinc.com/learn/programmable-logic/tutorials/nexys-4-basic-user-demo/start

You have to remember, when you are required to connect the switches (and other I/O pints to your

design’s signals, just add the VHDL (or Verilog) code that assigns each LED signal to each switch:

LEDS[3:0] <= SWITCHES[3:0] (this is NOT the code!)

Save your file after entering the code. If you do not know how to assign signals, refresh your

VHDL skills by re-reading the tutorials and checking the website!!!

To compile and synthesize your VHDL code, make sure the SOURCE_FILE.vhd file is highlighted

in the Sources in Project window, expand the Synthesize item in the Processes for Source window

and double-click Check Syntax.

The console should not display any errors under the HDL Compilation section, and a should

appear next to Check Syntax. If you get compilation errors, resolve them (ask for help if necessary)

before continuing.

Accessing the I/Os

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The Nexys4 DDR is compatible with Xilinx’s new high-performance Vivado® Design Suite as well

as the ISE® toolset, which includes ChipScope™ and EDK. Xilinx offers free WebPACK™

versions of these toolsets, so designs can be implemented at no additional cost. The Nexys4 DDR is

not supported by the Digilent Adept Utility.

Callout Component Description Callout Component Description

1 Power select jumper and battery header 13 FPGA configuration reset button

2 Shared UART/ JTAG USB port 14 CPU reset button (for soft cores)

3 External configuration jumper (SD / USB) 15 Analog signal Pmod port (XADC)

4 Pmod port(s) 16 Programming mode jumper

5 Microphone 17 Audio connector

6 Power supply test point(s) 18 VGA connector

7 LEDs (16) 19 FPGA programming done LED

8 Slide switches 20 Ethernet connector

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9 Eight digit 7-seg display 21 USB host connector

10 JTAG port for (optional) external cable 22 PIC24 programming port (factory use)

11 Five pushbuttons 23 Power switch

12 Temperature sensor 24 Power jack

A growing collection of board support IP, reference designs, and add-on boards are available on the

Digilent website. See the Nexys4 DDR page at www.digilentinc.com for more information.

1.1 Migrating from Nexys4

The Nexys4 DDR is an incremental update to the Nexys4 board. The major improvement is the

replacement of the 16 MiB CellularRAM with a 128 MiB DDR2 SDRAM memory. Digilent will

provide a VHDL reference module that wraps the complexity of a DDR2 controller and is backwards

compatible with the asynchronous SRAM interface of the CellularRAM, with certain limitations. See

the Nexys4 DDR page at www.digilentinc.com for updates. Furthermore, to accommodate the new

memory, the pin-out of the FPGA banks has changed as well. The constraints file of existing projects

will need to be updated.

The audio output (AUD_PWM) needs to be driven open-drain as opposed to push-pull on the

Nexys4.

2 Power Supplies

The Nexys4 DDR board can receive power from the Digilent USB-JTAG port (J6) or from an

external power supply. Jumper JP3 (near the power jack) determines which source is used.

All Nexys4 DDR power supplies can be turned on and off by a single logic-level power switch

(SW16). A power-good LED (LD22), driven by the “power good” output of the ADP2118 supply,

indicates that the supplies are turned on and operating normally. An overview of the Nexys4 DDR

power circuit is shown in Figure 2.

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The USB port can deliver enough power for the vast majority of designs. Our out-of-box demo

draws ~400mA of current from the 5V input rail. A few demanding applications, including any that

drive multiple peripheral boards, might require more power than the USB port can provide. Also,

some applications may need to run without being connected to a PC’s USB port. In these instances,

an external power supply or battery pack can be used.

An external power supply can be used by plugging into to the power jack (JP3) and setting jumper

J13 to “wall”. The supply must use a coax, center-positive 2.1mm internal-diameter plug, and deliver

4.5VDC to 5.5VDC and at least 1A of current (i.e., at least 5W of power). Many suitable supplies

can be purchased from Digilent, through Digi-Key, or other catalog vendors.

An external battery pack can be used by connecting the battery’s positive terminal to the center pin

of JP3 and the negative terminal to the pin labeled J12, directly below JP3. Since the main regulator

on the Nexys4 DDR cannot accommodate input voltages over 5.5VDC, an external battery pack

must be limited to 5.5VDC. The minimum voltage of the battery pack depends on the application: if

the USB Host function (J5) is used, at least 4.6V needs to be provided. In other cases, the minimum

voltage is 3.6V.

Voltage regulator circuits from Analog Devices create the required 3.3V, 1.8V, and 1.0V supplies

from the main power input. Table 1 provides additional information. Typical currents depend

strongly on FPGA configuration and the values provided are typical of medium size/speed designs.

Table 1. Nexys4 DDR power supplies.

Supply Circuits Device Current

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(max/typical)

3.3V FPGA I/O, USB ports, Clocks, RAM I/O, Ethernet, SD

slot, Sensors, Flash

IC17:

ADP2118 3A/0.1 to 1.5A

1.0V FPGA Core IC22:

ADP2118 3A/ 0.2 to 1.3A

1.8V DDR2, FPGA Auxiliary and RAM IC23:

ADP2118 0.8A/ 0.5A

2.1 Protection

The Nexys4 DDR features overcurrent and overvoltage protection on the input power rail. A 3.5A

fuse (R287) and a 5V Zener diode (D16) provide a non-resettable protection for other on-board

integrated circuits, as displayed in Figure 2. Applying power outside of the specs outlined in this

document is not covered by warranty. If this happens, either or both might get permanently damaged.

The damaged parts are not user-replaceable.

3 FPGA Configuration

After power-on, the Artix-7 FPGA must be configured (or programmed) before it can perform any

functions. You can configure the FPGA in one of four ways:

1. A PC can use the Digilent USB-JTAG circuitry (portJ6, labeled “PROG”) to program the FPGA any time the power is on.

2. A file stored in the nonvolatile serial (SPI) flash device can be transferred to the FPGA using the SPI port.

3. A programming file can be transferred to the FPGA from a micro SD card. 4. A programming file can be transferred from a USB memory stick attached to the USB HID

port.

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Figure 3 shows the different options available for configuring the FPGA. An on-board “mode”

jumper (JP1) and a media selection jumper (JP2) select between the programming modes.

The FPGA configuration data is stored in files called bitstreams that have the .bit file extension. The

ISE or Vivado software from Xilinx can create bitstreams from VHDL, Verilog®, or schematic-

based source files (in the ISE toolset, EDK is used for MicroBlaze™ embedded processor-based

designs).

Bitstreams are stored in SRAM-based memory cells within the FPGA. This data defines the FPGA’s

logic functions and circuit connections, and it remains valid until it is erased by removing board

power, by pressing the reset button attached to the PROG input, or by writing a new configuration

file using the JTAG port.

An Artix-7 100T bitstream is typically 30,606,304 bits and can take a long time to transfer. The time

it takes to program the Nexys4 can be decreased by compressing the bitstream before programming,

and then allowing the FPGA to decompress the bitstream itself during configuration. Depending on

design complexity, compression ratios of 10x can be achieved. Bitstream compression can be

enabled within the Xilinx tools (ISE or Vivado) to occur during generation. For instructions on how

to do this, consult the Xilinx documentation for the toolset being used. After being successfully

programmed, the FPGA will cause the “DONE” LED to illuminate. Pressing the “PROG” button at

any time will reset the configuration memory in the FPGA. After being reset, the FPGA will

immediately attempt to reprogram itself from whatever method has been selected by the

programming mode jumpers.

The following sections provide greater detail about programming the Nexys4 DDR using the

different methods available.

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3.1 JTAG Configuration

The Xilinx tools typically communicate with FPGAs using the Test Access Port and Boundary-Scan

Architecture, commonly referred to as JTAG. During JTAG programming, a .bit file is transferred

from the PC to the FPGA using the onboard Digilent USB-JTAG circuitry (port J6) or an external

JTAG programmer, such as the Digilent JTAG-HS2, attached to port J10. You can perform JTAG

programming any time after the Nexys4 DDR has been powered on, regardless of what the mode

jumper (JP1) is set to. If the FPGA is already configured, then the existing configuration is

overwritten with the bitstream being transmitted over JTAG. Setting the mode jumper to the JTAG

setting (seen in Figure 3) is useful to prevent the FPGA from being configured from any other

bitstream source until a JTAG programming occurs.

Programming the Nexys4 DDR with an uncompressed bitstream using the on-board USB-JTAG

circuitry usually takes around five seconds. JTAG programming can be done using the hardware

server in Vivado or the iMPACT tool included with ISE and the Lab Tools version of Vivado. The

demonstration project available at www.digilentinc.com gives an in-depth tutorial on how to

program your board.

3.2 Quad-SPI Configuration

Since the FPGA on the Nexys4 DDR is volatile, it relies on the Quad-SPI flash memory to store the

configuration between power cycles. This configuration mode is called Master SPI. The blank FPGA

takes the role of master and reads the configuration file out of the flash device upon power-up. To

that effect, a configuration file needs to be downloaded first to the flash. When programming a

nonvolatile flash device, a bitstream file is transferred to the flash in a two-step process. First, the

FPGA is programmed with a circuit that can program flash devices, and then data is transferred to

the flash device via the FPGA circuit (this complexity is hidden from the user by the Xilinx tools).

This is called indirect programming. After the flash device has been programmed, it can

automatically configure the FPGA at a subsequent power-on or reset event as determined by the

mode jumper setting (see Figure 3). Programming files stored in the flash device will remain until

they are overwritten, regardless of power-cycle events.

Programming the flash can take as long as four to five minutes, which is mostly due to the lengthy

erase process inherent to the memory technology. Once written however, FPGA configuration can be

very fast—less than a second. Bitstream compression, SPI bus width, and configuration rate are

factors controlled by the Xilinx tools that can affect configuration speed. The Nexys4 DDR supports

x1, x2, and x4 bus widths and data rates of up to 50 MHz for Quad-SPI programming.

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Quad-SPI programming can be done using the iMPACT tool included with ISE or the Lab Tools

version of Vivado.

3.3 USB Host and Micro SD Programming

You can program the FPGA from a pen drive attached to the USB Host port (J5) or a microSD card

inserted into J1 by doing the following:

1. Format the storage device (Pen drive or microSD card) with a FAT32 file system. 2. Place a single .bit configuration file in the root directory of the storage device. 3. Attach the storage device to the Nexys4 DDR. 4. Set the JP1 Programming Mode jumper on the Nexys4 DDR to “USB/SD”. 5. Select the desired storage device using JP2. 6. Push the PROG button or power-cycle the Nexys4 DDR.

The FPGA will automatically configure with the .bit file on the selected storage device. Any .bit files

that are not built for the proper Artix-7 device will be rejected by the FPGA.

The Auxiliary Function Status, or “BUSY” LED, gives visual feedback on the state of the

configuration process when the FPGA is not yet programmed:

• When steadily lit, the auxiliary microcontroller is either booting up or currently reading the configuration medium (microSD or pen drive) and downloading a bitstream to the FPGA.

• A slow pulse means the microcontroller is waiting for a configuration medium to be plugged in.

• In case of an error during configuration, the LED will blink rapidly.

When the FPGA has been successfully configured, the behavior of the LED is application-specific.

For example, if a USB keyboard is plugged in, a rapid blink will signal the receipt of an HID input

report from the keyboard.

4 Memory

The Nexys4 DDR board contains two external memories: a 1Gib (128MiB) DDR2 SDRAM and a

128Mib (16MiB) non-volatile serial Flash device. The DDR2 modules are integrated on-board and

connect to the FPGA using the industry standard interface. The serial Flash is on a dedicated quad-

mode (x4) SPI bus. The connections and pin assignments between the FPGA and external memories

are shown below.

4.1 DDR2

The Nexys4 DDR includes one Micron MT47H64M16HR-25:H DDR2 memory component,

creating a single rank, 16-bit wide interface. It is routed to a 1.8V-powered HR (High Range) FPGA

bank with 50 ohm controlled single-ended trace impedance. 50 ohm internal terminations in the

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FPGA are used to match the trace characteristics. Similarly, on the memory side, on-die terminations

(ODT) are used for impedance matching.

For proper operation of the memory, a memory controller and physical layer (PHY) interface needs

to be included in the FPGA design. There are two recommended ways to do that, which are outlined

below and differ in complexity and design flexibility.

The straightforward way is to use the Digilent-provided DDR-to-SRAM adapter module which

instantiates the memory controller and uses an asynchronous SRAM bus for interfacing with user

logic. This module provides backward compatibility with projects written for older Nexys-line

boards featuring a CellularRAM instead of DDR2. It trades memory bandwidth for simplicity.

More advanced users or those who wish to learn more about DDR SDRAM technology may want to

use the Xilinx 7-series memory interface solutions core generated by the MIG (Memory Interface

Generator) Wizard. Depending on the tool used (ISE, EDK or Vivado), the MIG Wizard can

generate a native FIFO-style or an AXI4 interface to connect to user logic. This workflow allows the

customization of several DDR parameters optimized for the particular application. Table 2 below

lists the MIG Wizard settings optimized for the Nexys4 DDR.

Table 2. DDR2 settings for the Nexys4 DDR.

Setting Value

Memory type DDR2 SDRAM

Max. clock period 3000ps (667Mbps data rate)

Recommended clock period (for easy clock generation) 3077ps (650Mbps data rate)

Memory part MT47H64M16HR-25E

Data width 16

Data mask Enabled

Chip Select pin Enabled

Rtt (nominal) – On-die termination 50ohms

Internal Vref Enabled

Internal termination impedance 50ohms

Although the FPGA, memory IC, and the board itself are capable of the maximum data rate of

667Mbps, the limitations in the clock generation primitives restrict the clock frequencies that can be

generated from the 100 MHz system clock. Thus, for simplicity, the next highest data rate of

650Mbps is recommended.

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The MIG Wizard will require the fixed pin-out of the memory signals to be entered and validated

before generating the IP core. For your convenience, an importable UCF file is provided on the

Digilent website to speed up the process.

For more details on the Xilinx memory interface solutions, refer to the 7 Series FPGAs Memory

Interface Solutions User Guide (ug586)¹.

4.2 Quad-SPI Flash

FPGA configuration files can be written to the Quad-SPI Flash (Spansion part number S25FL128S),

and mode settings are available to cause the FPGA to automatically read a configuration from this

device at power on. An Artix-7 100T configuration file requires just less than four MiB (mebibyte)

of memory, leaving about 77% of the flash device available for user data. Or, if the FPGA is getting

configured from another source, the whole memory can be used for custom data.

The contents of the memory can be manipulated by issuing certain commands on the SPI bus. The

implementation of this protocol is outside the scope of this document. All signals in the SPI bus

except SCK are general-purpose user I/O pins after FPGA configuration. SCK is an exception

because it remains a dedicated pin even after configuration. Access to this pin is provided through a

special FPGA primitive called STARTUPE2.

NOTE: Refer to the manufacturer’s data sheets² and Xilinx user guides³ for more information.

¹http://www.xilinx.com/support/documentation/ip_documentation/mig_7series/v2_1/ug586_7Series_

MIS.pdf ²https://www.spansion.com/Support/Datasheets/S25FL032P_00.pdf

³http://www.xilinx.com/support/documentation/user_guides/ug470_7Series_Config.pdf

5 Ethernet PHY

The Nexys4 DDR board includes an SMSC 10/100 Ethernet PHY (SMSC part number LAN8720A)

paired with an RJ-45 Ethernet jack with integrated magnetics. The SMSC PHY uses the RMII

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interface and supports 10/100 Mb/s. Figure 5 illustrates the pin connections between the Artix-7 and

the Ethernet PHY. At power-on reset, the PHY is set to the following defaults:

• RMII mode interface • Auto-negotiation enabled, advertising all 10/100 mode capable • PHY address=00001

Two on-board LEDs (LD23 = LED2, LD24 = LED1) connected to the PHY provide link status and

data activity feedback. See the PHY datasheet for details.

EDK-based designs can access the PHY using either the axi_ethernetlite (AXI EthernetLite) IP core

or the axi_ethernet (Tri Mode Ethernet MAC) IP core. A mii_to_rmii core (Ethernet PHY MII to

Reduced MII) needs to be inserted to convert the MAC interface from MII to RMII. Also, a 50 MHz

clock needs to be generated for the mii_to_rmii core and the CLKIN pin of the external PHY. To

account for skew introduced by the mii_to_rmii core, generate each clock individually, with the

external PHY clock having a 45 degree phase shift relative to the mii_to_rmii Ref_Clk. An EDK

demonstration project that properly uses the Ethernet PHY can be found on the Nexys4 DDR product

page at www.digilentinc.com.

ISE designs can use the IP Core Generator wizard to create an Ethernet MAC controller IP core.

NOTE: Refer to the LAN8720A data sheet¹ for further information.

¹http://ww1.microchip.com/downloads/en/DeviceDoc/8720a.pdf

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6 Oscillators/Clocks

The Nexys4 DDR board includes a single 100 MHz crystal oscillator connected to pin E3 (E3 is a

MRCC input on bank 35). The input clock can drive MMCMs or PLLs to generate clocks of various

frequencies and with known phase relationships that may be needed throughout a design. Some rules

restrict which MMCMs and PLLs may be driven by the 100 MHz input clock. For a full description

of these rules and of the capabilities of the Artix-7 clocking resources, refer to the “7 Series FPGAs

Clocking Resources User Guide” available from Xilinx.

Xilinx offers the Clocking Wizard IP core to help users generate the different clocks required for a

specific design. This wizard will properly instantiate the needed MMCMs and PLLs based on the

desired frequencies and phase relationships specified by the user. The wizard will then output an

easy-to-use wrapper component around these clocking resources that can be inserted into the user’s

design. The clocking wizard can be accessed from within the Project Navigator or Core Generator

tools.

7 USB-UART Bridge (Serial Port)

The Nexys4 DDR includes an FTDI FT2232HQ USB-UART bridge (attached to connector J6) that

allows you use PC applications to communicate with the board using standard Windows COM port

commands. Free USB-COM port drivers, available from www.ftdichip.com under the “Virtual Com

Port” or VCP heading, convert USB packets to UART/serial port data. Serial port data is exchanged

with the FPGA using a two-wire serial port (TXD/RXD) and optional hardware flow control

(RTS/CTS). After the drivers are installed, I/O commands can be used from the PC directed to the

COM port to produce serial data traffic on the C4 and D4 FPGA pins.

Two on-board status LEDs provide visual feedback on traffic flowing through the port: the transmit

LED (LD20) and the receive LED (LD19). Signal names that imply direction are from the point-of-

view of the DTE (Data Terminal Equipment), in this case the PC.

The FT2232HQ is also used as the controller for the Digilent USB-JTAG circuitry, but the USB-

UART and USB-JTAG functions behave entirely independent of one another. Programmers

interested in using the UART functionality of the FT2232 within their design do not need to worry

about the JTAG circuitry interfering with the UART data transfers, and vice-versa. The combination

of these two features into a single device allows the Nexys4 DDR to be programmed, communicated

with via UART, and powered from a computer attached with a single Micro USB cable.

The connections between the FT2232HQ and the Artix-7 are shown in Figure 6.

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8 USB HID Host

The Auxiliary Function microcontroller (Microchip PIC24FJ128) provides the Nexys4 DDR with

USB Embedded HID host capability. After power-up, the microcontroller is in configuration mode,

either downloading a bitstream to the FPGA, or waiting to be programmed from other sources. Once

the FPGA is programmed, the microcontroller switches to application mode, which is USB HID Host

in this case. Firmware in the microcontroller can drive a mouse or a keyboard attached to the type A

USB connector at J5 labeled “USB Host”. Hub support is not currently available, so only a single

mouse or a single keyboard can be used. Only keyboards and mice supporting the Boot HID

interface are supported. The PIC24 drives several signals into the FPGA – two are used to implement

a standard PS/2 interface for communication with a mouse or keyboard, and the others are connected

to the FPGA’s two-wire serial programming port, so the FPGA can be programmed from a file

stored on a USB pen drive or microSD card.

8.1 HID Controller

The Auxiliary Function microcontroller hides the USB HID protocol from the FPGA and emulates

an old-style PS/2 bus. The microcontroller behaves just like a PS/2 keyboard or mouse would. This

means new designs can re-use existing PS/2 IP cores. Mice and keyboards that use the PS/2 protocol

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use a two-wire serial bus (clock and data) to communicate with a host. On the Nexys4 DDR, the

microcontroller emulates a PS/2 device while the FPGA plays the role of the host. Both the mouse

and the keyboard use 11-bit words that include a start bit, data byte (LSB first), odd parity, and stop

bit, but the data packets are organized differently, and the keyboard interface allows bi-directional

data transfers (so the host device can illuminate state LEDs on the keyboard). Bus timings are shown

in Figure 8.

The clock and data signals are only driven when data transfers occur; otherwise, they are held in the

idle state at high-impedance (open-drain drivers). This requires that when the PS/2 signals are used

in a design, internal pull-ups must be enabled in the FPGA on the data and clock pins. The clock

signal is normally driven by the device, but may be held low by the host in special cases. The timings

define signal requirements for mouse-to-host communications and bi-directional keyboard

communications. A PS/2 interface circuit can be implemented in the FPGA to create a keyboard or

mouse interface.

When a keyboard or mouse is connected to the Nexys4 DDR, a “self-test passed” command (0xAA)

is sent to the host. After this, commands may be issued to the device. Since both the keyboard and

the mouse use the same PS/2 port, one can tell the type of device connected using the device ID. This

ID can be read by issuing a Read ID command (0xF2). Also, a mouse sends its ID (0x00) right after

the “self-test passed” command, which distinguishes it from a keyboard.

8.2 Keyboard

PS/2-style keyboards use scan codes to communicate key press data. Each key is assigned a code that

is sent whenever the key is pressed. If the key is held down, the scan code will be sent repeatedly

about once every 100ms. When a key is released, an F0 key-up code is sent, followed by the scan

code of the released key. If a key can be shifted to produce a new character (like a capital letter), then

a shift character is sent in addition to the scan code and the host must determine which ASCII

character to use. Some keys, called extended keys, send an E0 ahead of the scan code (and they may

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send more than one scan code). When an extended key is released, an E0 F0 key-up code is sent,

followed by the scan code. Scan codes for most keys are shown in Figure 9.

A host device can also send data to the keyboard. Table 3 shows a list of some common commands a

host might send.

The keyboard can send data to the host only when both the data and clock lines are high (or idle).

Because the host is the bus master, the keyboard must check to see whether the host is sending data

before driving the bus. To facilitate this, the clock line is used as a “clear to send” signal. If the host

drives the clock line low, the keyboard must not send any data until the clock is released. The

keyboard sends data to the host in 11-bit words that contain a ‘0’ start bit, followed by 8-bits of scan

code (LSB first), followed by an odd parity bit, and terminated with a ‘1’ stop bit. The keyboard

generates 11 clock transitions (at 20 to 30 KHz) when the data is sent, and data is valid on the falling

edge of the clock.

Table 3. Keyboard commands.

Command Action

ED

Set Num Lock, Caps Lock, and Scroll Lock LEDs. Keyboard returns FA after receiving

ED, then host sends a byte to set LED status: bit 0 sets Scroll Lock, bit 1 sets Num Lock,

and bit 2 sets Caps lock. Bits 3 to 7 are ignored.

EE Echo (test). Keyboard returns EE after receiving EE

F3 Set scan code repeat rate. Keyboard returns F3 on receiving FA, then host sends second

byte to set the repeat rate.

FE Resend. FE directs keyboard to re-send most recent scan code.

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FF Reset. Resets the keyboard.

8.3 Mouse

Once entered in stream mode and data reporting is enabled, the mouse outputs a clock and data

signal when it is moved; otherwise, these signals remain at logic ‘1.’ Each time the mouse is moved,

three 11-bit words are sent from the mouse to the host device, as shown in Figure 10. Each of the 11-

bit words contains a ‘0’ start bit, followed by 8 bits of data (LSB first), followed by an odd parity bit,

and terminated with a ‘1’ stop bit. Thus, each data transmission contains 33 bits, where bits 0, 11,

and 22 are ‘0’ start bits, and bits 11, 21, and 33 are ‘1’ stop bits. The three 8-bit data fields contain

movement data, as shown in Figure 10. Data is valid at the falling edge of the clock, and the clock

period is 20 to 30 KHz.

The mouse assumes a relative coordinate system wherein moving the mouse to the right generates a

positive number in the X field, and moving to the left generates a negative number. Likewise,

moving the mouse up generates a positive number in the Y field, and moving down represents a

negative number (the XS and YS bits in the status byte are the sign bits – a ‘1’ indicates a negative

number). The magnitude of the X and Y numbers represent the rate of mouse movement; the larger

the number, the faster the mouse is moving (the XV and YV bits in the status byte are movement

overflow indicators. A ‘1’ means overflow has occurred). If the mouse moves continuously, the 33-

bit transmissions are repeated every 50ms or so. The L and R fields in the status byte indicate Left

and Right button presses (a ‘1’ indicates the button is being pressed).

The microcontroller also supports Microsoft® IntelliMouse®-type extensions for reporting back a

third axis representing the mouse wheel, as shown in Table 4.

Table 4. Microsoft IntelliMouse-type extensions, commands, and actions.

Command Action

EA Set stream mode. The mouse responds with “acknowledge” (0xFA) then resets its

movement counters and enters stream mode.

F4

Enable data reporting. The mouse responds with “acknowledge” (0xFA) then enables

data reporting and resets its movement counters. This command only affects behavior in

stream mode. Once issued, mouse movement will automatically generate a data packet.

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F5 Disable data reporting. The mouse responds with “acknowledge” (0xFA) then disables

data reporting and resets its movement counters.

F3

Set mouse sample rate. The mouse responds with “acknowledge” (0xFA) then reads one

more byte from the host. This byte is then saved as the new sample rate, and a new

“acknowledge” packet is issued.

FE Resend. FE directs mouse to re-send last packet.

FF Reset. The mouse responds with “acknowledge” (0xFA) then enters reset mode.

9 VGA Port

The Nexys4 DDR board uses 14 FPGA signals to create a VGA port with 4 bits-per-color and the

two standard sync signals (HS – Horizontal Sync, and VS – Vertical Sync). The color signals use

resistor-divider circuits that work in conjunction with the 75-ohm termination resistance of the VGA

display to create 16 signal levels each on the red, green, and blue VGA signals. This circuit, shown

in Figure 11, produces video color signals that proceed in equal increments between 0V (fully off)

and 0.7V (fully on). Using this circuit, 4096 different colors can be displayed, one for each unique

12-bit pattern. A video controller circuit must be created in the FPGA to drive the sync and color

signals with the correct timing in order to produce a working display system.

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9.1 VGA System Timing

VGA signal timings are specified, published, copyrighted, and sold by the VESA® organization

(www.vesa.org). The following VGA system timing information is provided as an example of how a

VGA monitor might be driven in 640 by 480 mode.

NOTE: For more precise information, or for information on other VGA frequencies, refer to

documentation available at the VESA website.

CRT-based VGA displays use amplitude-modulated moving electron beams (or cathode rays) to

display information on a phosphor-coated screen. LCD displays use an array of switches that can

impose a voltage across a small amount of liquid crystal, thereby changing light permittivity through

the crystal on a pixel-by-pixel basis. Although the following description is limited to CRT displays,

LCD displays have evolved to use the same signal timings as CRT displays (so the “signals”

discussion below pertains to both CRTs and LCDs). Color CRT displays use three electron beams

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(one for red, one for blue, and one for green) to energize the phosphor that coats the inner side of the

display end of a cathode ray tube (see Figure 12).

Electron beams emanate from “electron guns,” which are finely-pointed heated cathodes placed in

close proximity to a positively charged annular plate called a “grid.” The electrostatic force imposed

by the grid pulls rays of energized electrons from the cathodes, and those rays are fed by the current

that flows into the cathodes. These particle rays are initially accelerated towards the grid, but they

soon fall under the influence of the much larger electrostatic force that results from the entire

phosphor-coated display surface of the CRT being charged to 20kV (or more). The rays are focused

to a fine beam as they pass through the center of the grids, and then they accelerate to impact on the

phosphor-coated display surface. The phosphor surface glows brightly at the impact point, and it

continues to glow for several hundred microseconds after the beam is removed. The larger the

current fed into the cathode, the brighter the phosphor will glow.

Between the grid and the display surface, the beam passes through the neck of the CRT where two

coils of wire produce orthogonal electromagnetic fields. Because cathode rays are composed of

charged particles (electrons), they can be deflected by these magnetic fields. Current waveforms are

passed through the coils to produce magnetic fields that interact with the cathode rays and cause

them to transverse the display surface in a “raster” pattern, horizontally from left to right and

vertically from top to bottom, as shown in Figure 13. As the cathode ray moves over the surface of

the display, the current sent to the electron guns can be increased or decreased to change the

brightness of the display at the cathode ray impact point.

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Information is only displayed when the beam is moving in the “forward” direction (left to right and

top to bottom), and not during the time the beam is reset back to the left or top edge of the display.

Much of the potential display time is therefore lost in “blanking” periods when the beam is reset and

stabilized to begin a new horizontal or vertical display pass. The size of the beams, the frequency at

which the beam can be traced across the display, and the frequency at which the electron beam can

be modulated determine the display resolution.

Modern VGA displays can accommodate different resolutions, and a VGA controller circuit dictates

the resolution by producing timing signals to control the raster patterns. The controller must produce

synchronizing pulses at 3.3V (or 5V) to set the frequency at which current flows through the

deflection coils, and it must ensure that video data is applied to the electron guns at the correct time.

Raster video displays define a number of “rows” that corresponds to the number of horizontal passes

the cathode makes over the display area, and a number of “columns” that corresponds to an area on

each row that is assigned to one “picture element,” or pixel. Typical displays use from 240 to 1200

rows and from 320 to 1600 columns. The overall size of a display and the number of rows and

columns determines the size of each pixel.

Video data typically comes from a video refresh memory; with one or more bytes assigned to each

pixel location (the Nexys4 DDR uses 12 bits per pixel). The controller must index into video

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memory as the beams move across the display, and retrieve and apply video data to the display at

precisely the time the electron beam is moving across a given pixel.

A VGA controller circuit must generate the HS and VS timings signals and coordinate the delivery

of video data based on the pixel clock. The pixel clock defines the time available to display one pixel

of information. The VS signal defines the “refresh” frequency of the display, or the frequency at

which all information on the display is redrawn. The minimum refresh frequency is a function of the

display’s phosphor and electron beam intensity, with practical refresh frequencies falling in the 50Hz

to 120Hz range. The number of lines to be displayed at a given refresh frequency defines the

horizontal “retrace” frequency. For a 640-pixel by 480-row display using a 25 MHz pixel clock and

60 +/-1Hz refresh, the signal timings shown in Figure 14 can be derived. Timings for sync pulse

width and front and back porch intervals (porch intervals are the pre- and post-sync pulse times

during which information cannot be displayed) are based on observations taken from actual VGA

displays.

A VGA controller circuit, such as the one diagrammed in Figure 15, decodes the output of a

horizontal-sync counter driven by the pixel clock to generate HS signal timings. You can use this

counter to locate any pixel location on a given row. Likewise, the output of a vertical-sync counter

that increments with each HS pulse can be used to generate VS signal timings, and you can use this

counter to locate any given row. These two continually running counters can be used to form an

address into video RAM. No time relationship between the onset of the HS pulse and the onset of the

VS pulse is specified, so you can arrange the counters to easily form video RAM addresses, or to

minimize decoding logic for sync pulse generation.

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10 Basic I/O

The Nexys4 DDR board includes two tri-color LEDs, sixteen slide switches, six push buttons,

sixteen individual LEDs, and an eight-digit seven-segment display, as shown in Figure 16. The

pushbuttons and slide switches are connected to the FPGA via series resistors to prevent damage

from inadvertent short circuits (a short circuit could occur if an FPGA pin assigned to a pushbutton

or slide switch was inadvertently defined as an output). The five pushbuttons arranged in a plus-sign

configuration are “momentary” switches that normally generate a low output when they are at rest,

and a high output only when they are pressed. The red pushbutton labeled “CPU RESET,” on the

other hand, generates a high output when at rest and a low output when pressed. The CPU RESET

button is intended to be used in EDK designs to reset the processor, but you can also use it as a

general purpose pushbutton. Slide switches generate constant high or low inputs depending on their

position.

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The sixteen individual high-efficiency LEDs are anode-connected to the FPGA via 330-ohm

resistors, so they will turn on when a logic high voltage is applied to their respective I/O pin.

Additional LEDs that are not user-accessible indicate power-on, FPGA programming status, and

USB and Ethernet port status.

10.1 Seven-Segment Display

The Nexys4 DDR board contains two four-digit common anode seven-segment LED displays,

configured to behave like a single eight-digit display. Each of the eight digits is composed of seven

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segments arranged in a “figure 8” pattern, with an LED embedded in each segment. Segment LEDs

can be individually illuminated, so any one of 128 patterns can be displayed on a digit by

illuminating certain LED segments and leaving the others dark, as shown in Figure 17. Of these 128

possible patterns, the ten corresponding to the decimal digits are the most useful.

The anodes of the seven LEDs forming each digit are tied together into one “common anode” circuit

node, but the LED cathodes remain separate, as shown in Fig 18. The common anode signals are

available as eight “digit enable” input signals to the 8-digit display. The cathodes of similar segments

on all four displays are connected into seven circuit nodes labeled CA through CG. For example, the

eight “D” cathodes from the eight digits are grouped together into a single circuit node called “CD.”

These seven cathode signals are available as inputs to the 8-digit display. This signal connection

scheme creates a multiplexed display, where the cathode signals are common to all digits but they

can only illuminate the segments of the digit whose corresponding anode signal is asserted.

To illuminate a segment, the anode should be driven high while the cathode is driven low. However,

since the Nexys4 DDR uses transistors to drive enough current into the common anode point, the

anode enables are inverted. Therefore, both the AN0..7 and the CA..G/DP signals are driven low

when active.

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A scanning display controller circuit can be used to show an eight-digit number on this display. This

circuit drives the anode signals and corresponding cathode patterns of each digit in a repeating,

continuous succession at an update rate that is faster than the human eye can detect. Each digit is

illuminated just one-eighth of the time, but because the eye cannot perceive the darkening of a digit

before it is illuminated again, the digit appears continuously illuminated. If the update, or “refresh”,

rate is slowed to around 45Hz, a flicker can be noticed in the display.

For each of the four digits to appear bright and continuously illuminated, all eight digits should be

driven once every 1 to 16ms, for a refresh frequency of about 1 KHz to 60Hz. For example, in a

62.5Hz refresh scheme, the entire display would be refreshed once every 16ms, and each digit would

be illuminated for 1/8 of the refresh cycle, or 2ms. The controller must drive low the cathodes with

the correct pattern when the corresponding anode signal is driven high. To illustrate the process, if

AN0 is asserted while CB and CC are asserted, then a “1” will be displayed in digit position 1. Then,

if AN1 is asserted while CA, CB, and CC are asserted, a “7” will be displayed in digit position 2. If

AN0, CB, and CC are driven for 4ms, and then AN1, CA, CB, and CC are driven for 4ms in an

endless succession, the display will show “71” in the first two digits. An example timing diagram for

a four-digit controller is shown in Figure 19.

10.2 Tri-Color LED

The Nexys4 DDR board contains two tri-color LEDs. Each tri-color LED has three input signals that

drive the cathodes of three smaller internal LEDs: one red, one blue, and one green. Driving the

signal corresponding to one of these colors high will illuminate the internal LED. The input signals

are driven by the FPGA through a transistor, which inverts the signals. Therefore, to light up the tri-

color LED, the corresponding signals need to be driven high. The tri-color LED will emit a color

dependent on the combination of internal LEDs that are currently being illuminated. For example, if

the red and blue signals are driven high, and green is driven low, the tri-color LED will emit a purple

color.

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Note: Digilent strongly recommends the use of pulse-width modulation (PWM) when driving the tri-

color LEDs (for information on PWM, see section 15.1 Pulse Density Modulation (PDM)). Driving

any of the inputs to a steady logic ‘1’ will result in the LED being illuminated at an uncomfortably

bright level. You can avoid this by ensuring that none of the tri-color signals are driven with more

than a 50% duty cycle. Using PWM also greatly expands the potential color palette of the tri-color

led. Individually adjusting the duty cycle of each color between 50% and 0% causes the different

colors to be illuminated at different intensities, allowing virtually any color to be displayed.

11 Pmod Ports

The Pmod ports are arranged in a 2×6 right-angle, and are 100-mil female connectors that mate with

standard 2×6 pin headers. Each 12-pin Pmod port provides two 3.3V VCC signals (pins 6 and 12),

two Ground signals (pins 5 and 11), and eight logic signals, as shown in Figure 20. The VCC and

Ground pins can deliver up to 1A of current. Pmod data signals are not matched pairs, and they are

routed using best-available tracks without impedance control or delay matching. Pin assignments for

the Pmod I/O connected to the FPGA are shown in Table 5.

Table 5. Nexys4 DDR Pmod pin assignments.

Pmod JA Pmod JB Pmod JC Pmod JD Pmod XDAC

JA1: C17 JB1: D14 JC1: K1 JD1: H4 JXADC1: A13 (AD3P)

JA2: D18 JB2: F16 JC2: F6 JD2: H1 JXADC2: A15 (AD10P)

JA3: E18 JB3: G16 JC3: J2 JD3: G1 JXADC3: B16 (AD2P)

JA4: G17 JB4: H14 JC4: G6 JD4: G3 JXADC4: B18 (AD11P)

JA7: D17 JB7: E16 JC7: E7 JD7: H2 JXADC7: A14 (AD3N)

JA8: E17 JB8: F13 JC8: J3 JD8: G4 JXADC8: A16 (AD10N)

JA9: F18 JB9: G13 JC9: J4 JD9: G2 JXADC9: B17 (AD2N)

JA10: G18 JB10: H16 JC10: E6 JD10: F3 JXADC10: A18 (AD11N)

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Digilent produces a large collection of Pmod accessory boards that can attach to the Pmod expansion

connectors to add ready-made functions like A/D’s, D/A’s, motor drivers, sensors, as well as other

functions. See www.digilentinc.com for more information.

11.1 Dual Analog/Digital Pmod

The on-board Pmod expansion connector labeled “JXADC” is wired to the auxiliary analog input

pins of the FPGA. Depending on the configuration, this connector can be used to input differential

analog signals to the analog-to-digital converter inside of the Artix-7 (XADC). Any or all pairs in the

connector can be configured either as analog input or digital input-output.

The Dual Analog/Digital Pmod on the Nexys4 DDR differs from the rest in the routing of its traces.

The eight data signals are grouped into four pairs, with the pairs routed closely coupled for better

analog noise immunity. Furthermore, each pair has a partially loaded anti-alias filter laid out on the

PCB. The filter does not have capacitors C60-C63. In designs where such filters are desired, the

capacitors can be manually loaded by the user.

NOTE: The coupled routing and the anti-alias filters might limit the data speeds when used for

digital signals.

The XADC core within the Artix-7 is a dual channel 12-bit analog-to-digital converter capable of

operating at 1 MSPS. Either channel can be driven by any of the auxiliary analog input pairs

connected to the JXADC header. The XADC core is controlled and accessed from a user design via

the Dynamic Reconfiguration Port (DRP). The DRP also provides access to voltage monitors that are

present on each of the FPGA’s power rails, and a temperature sensor that is internal to the FPGA.

For more information on using the XADC core, refer to the Xilinx document titled “7 Series FPGAs

and Zynq-7000 All Programmable SoC XADC Dual 12-Bit 1 MSPS Analog-to-Digital Converter.”

12 MicroSD Slot

The Nexys4 DDR provides a microSD slot for both FPGA configuration and user access. The on-

board Auxiliary Function microcontroller shares the SD card bus with the FPGA. Before the FPGA

is configured the microcontroller must have access to the SD card via SPI. Once a bit file is

downloaded to the FPGA (from any source), the microcontroller power cycles the SD slot and

relinquishes control of the bus. This enables any SD card in the slot to reset its internal state

machines and boot up in SD native bus mode. All of the SD pins on the FPGA are wired to support

full SD speeds in native interface mode, as shown in Figure 21. The SPI is also available, if needed.

Once control over the SD bus is passed from the microcontroller to the FPGA, the SD_RESET signal

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needs to be actively driven low by the FPGA to power the microSD card slot. For information on

implementing an SD card controller, refer to the SD card specification available at www.sdcard.org.

13 Temperature Sensor

The Nexys4 DDR includes an Analog Device ADT7420 temperature sensor. The sensor provides up

to 16-bit resolution with a typical accuracy better than 0.25 degrees Celsius. The interface between

the temperature sensor and FPGA is shown in Figure 22.

13.1 I²C Interface

The ADT7420 chip acts as a slave device using the industry standard I²C communication scheme. To

communicate with ADT7420 chip, the I²C master must specify a slave address (0x4B) and a flag

indicating whether the communication is a read (1) or a write (0). Once specifications are made for

communication, a data transfer takes place. For ADT7420, the data transfer should consist of the

address of the desired device register followed by the data to be written to the specified register. To

read from a register, the master must write the desired register address to the ADT7420, then send an

I²C restart condition, and send a new read request to the ADT7420. If the master does not generate a

restart condition prior to attempting the read, the value written to the address register will be reset to

0x00. As some registers store 16-bit values as 8-bit register pairs, the ADT7420 will automatically

increment the address register of the device when accessing certain registers, such as the temperature

registers and the threshold registers. This allows for the master to use a single read or write request to

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access both the low and high bytes of these registers. A complete listing of registers and their

behavior can be found in the ADT7420 datasheet available on the Analog Devices website.

13.2 Open Drain Outputs

The ADT7420 provides two open drain output signals to indicate when pre-set temperature

thresholds are reached. If the temperature leaves a range defined by registers TLOW (0x06:0x07)

and THIGH (0x04:0x05), the INT pin can be driven low or high based upon the configuration of the

device. Similarly, the CT pin can be driven low or high if the temperature exceeds a critical threshold

defined in TCRIT (0x08:0x09). Both of these pins need internal FPGA pull-ups when used.

For details on the electrical specifications and configuration of the INT and CT pins, refer to the

ADT7420 datasheet

13.3 Quick Start Operation

When the ADT7420 is powered up, it is in a mode that can be used as a simple temperature sensor

without any initial configuration. By default, the device address register points to the temperature

MSB register, so a two byte read without specifying a register will read the value of the temperature

register from the device. The first byte read back will be the most significant byte (MSB) of the

temperature data, and the second will be the least significant byte (LSB) of the data. These two bytes

form a two’s complement 16-bit integer. If the result is shifted to the right three bits and multiplied

by 0.0625, the resulting signed floating point value will be a temperature reading in degrees Celsius.

For information on reading and writing to the other registers of the device, as well as notes on the

accuracy of the temperature measurements, refer to the ADT7420 datasheet.

14 Accelerometer

The Nexys4 DDR includes an Analog Device ADXL362 accelerometer. The ADXL362 is a 3-axis

MEMS accelerometer that consumes less than 2μA at a 100Hz output data rate and 270nA when in

motion triggered wake-up mode. Unlike accelerometers that use power duty cycling to achieve low

power consumption, the ADXL362 does not alias input signals by under-sampling; it samples the

full bandwidth of the sensor at all data rates. The ADXL362 always provides 12-bit output

resolution; 8-bit formatted data is also provided for more efficient single-byte transfers when a lower

resolution is sufficient. Measurement ranges of ±2 g, ±4 g, and ±8 g are available with a resolution of

1 mg/LSB on the ±2 g range. The FPGA can talk with the ADXL362 via SPI interface. While the

ADXL362 is in Measurement Mode, it continuously measures and stores acceleration data in the X-

data, Y-data, and Z-data registers. The interface between the FPGA and accelerometer can be seen in

Figure 23.

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14.1 SPI Interface

The ADXL362 acts as a slave device using an SPI communication scheme. The recommended SPI

clock frequency ranges from 1 MHz to 5 MHz. The SPI operates in SPI mode 0 with CPOL = 0 and

CPHA = 0. All communications with the device must specify a register address and a flag that

indicate whether the communication is a read or a write. Actual data transfer always follows the

register address and communication flag. Device configuration can be performed by writing to the

control registers within the accelerometer. Access accelerometer data by reading the device registers.

For a full list of registers, their functionality, and communication specifications, refer to the

ADXL362 datasheet¹.

14.2 Interrupts

Several of the built-in functions of the ADXL362 can trigger interrupts that alert the host processor

of certain status conditions. Interrupts can be mapped to either (or both) of two interrupt pins (INT1,

INT2). Both of these pins require internal FPGA pull-ups when used. For more details about the

interrupts, see the ADXL362 datasheet.

¹http://www.analog.com/adxl362

15 Microphone

The Nexys4 DDR board includes an omnidirectional MEMS microphone. The microphone uses an

Analog Device ADMP421 chip which has a high signal to noise ratio (SNR) of 61dBA and high

sensitivity of -26 dBFS. It also has a flat frequency response ranging from 100Hz to 15 kHz. The

digitized audio is output in the pulse density modulated (PDM) format. The component architecture

is shown in Figure 24.

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15.1 Pulse Density Modulation (PDM)

PDM data connections are becoming more and more popular in portable audio applications, such as

cellphones and tablets. With PDM, two channels can be transmitted with only two wires. The

frequency of a PDM signal usually falls in the range of 1 MHz to 3 MHz. In a PDM bitstream, a 1

corresponds to a positive pulse and a 0 corresponds to a negative pulse. A run consisting of all ‘1’s

would correspond to the maximum positive value and a run of ‘0’s would correspond to the

minimum amplitude value. Figure 25 shows how a sine wave is represented in PDM signal.

A PDM signal is generated from an analog signal through a process called delta-sigma modulation.

A simple idealized circuit of delta-sigma modulator is shown in Figure 26.

Table 6. Sigma Delta Modulator with a 0.4Vdd input.

Sum Integrator Out Flip-flop Output

0.4-0=0.4 0+0.4=0.4 0

0.4-0=0.4 0.4+0.4=0.8 1

0.4-1=-0.6 0.8-0.6=0.2 0

0.4-0=0.4 0.2+0.4=0.6 1

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0.4-1=-0.6 0.6-0.6=0 0

0.4-0=0.4 0+0.4=0.4 0

0.4-0=0.4 0.4+0.4=0.8 1

0.4-1=-0.6 0.8-0.6=0.2 0

To keep things simple, assume that the analog input and digital output have the same voltage range

0~Vdd. The input of the flip-flop acts like a comparator (any signal above Vdd/2 is considered as ‘1’

and any input bellow Vdd/2 is considered ‘0’). The input of the integral circuit is the difference of

the input analog signal and the PDM signal of the previous clock cycle. The integral circuit then

integrates both of these inputs, and the output of the integral circuit is sampled by a D-Flip-flop.

Table 6 shows the function of the delta-sigma modulator with an input of 0.4Vdd.

Note that the average of the flip-flop output equals the value of the input analog signal. So in order to

get the value of analog input, all that is needed is a counter that counts the ‘1’s for a certain period of

time.

15.2 Microphone Digital Interface Timing

The clock input of the microphone can range from 1 MHz to 3.3 MHz based on the sampling rate

and data precision requirement of the applications. The L/R Select signal must be set to a valid level,

depending on which edge of the clock the data bit will be read. A low level on L/RSEL makes data

available on the rising edge of the clock, while a high level corresponds to the falling edge of the

clock, as shown in Figure 27.

The typical value of the clock frequency is 2.4 MHz. Assuming that the application requires 7-bit

precision and 24 KHz, there can be two counters that count 128 samples at 12 KHz, as shown in

Figure 28.

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16 Mono Audio Output

The on-board audio jack (J8) is driven by a Sallen-Key Butterworth Low-pass 4th Order Filter that

provides mono audio output. The circuit of the low-pass filter is shown in Figure 29. The input of the

filter (AUD_PWM) is connected to the FPGA pin A11. A digital input will typically be a pulse-

width modulated (PWM) or pulse density modulated (PDM) open-drain signal produced by the

FPGA. The signal needs to be driven low for logic ‘0’ and left in high-impedance for logic ‘1’. An

on-board pull-up resistor to a clean analog 3.3V rail will establish the proper voltage for logic ‘1’.

The low-pass filter on the input will act as a reconstruction filter to convert the pulse-width

modulated digital signal into an analog voltage on the audio jack output.

The frequency response of SK Butterworth Low-Pass Filter is shown in Figure 30. The AC analysis

of the circuit is done using NI Multisim 12.0.

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16.1 Pulse-Width Modulation

A pulse-width modulated (PWM) signal is a chain of pulses at some fixed frequency, with each pulse

potentially having a different width. This digital signal can be passed through a simple low-pass filter

that integrates the digital waveform to produce an analog voltage proportional to the average pulse-

width over some interval (the interval is determined by the 3dB cut-off frequency of the low-pass

filter and the pulse frequency). For example, if the pulses are high for an average of 10% of the

available pulse period, then an integrator will produce an analog value that is 10% of the Vdd

voltage. Figure 31 shows a waveform represented as a PWM signal.

The PWM signal must be integrated to define an analog voltage. The low-pass filter 3dB frequency

should be an order of magnitude lower than the PWM frequency, so that signal energy at the PWM

frequency is filtered from the signal. For example, if an audio signal must contain up to 5 KHz of

frequency information, then the PWM frequency should be at least 50 KHz (and preferably even

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higher). In general, in terms of analog signal fidelity, the higher the PWM frequency, the better.

Figure 32 shows a representation of a PWM integrator producing an output voltage by integrating the

pulse train. Note the steady-state filter output signal amplitude ratio to Vdd is the same as the pulse-

width duty cycle (duty cycle is defined as pulse-high time divided by pulse-window time).

17 Built-In Self-Test

A demonstration configuration is loaded into the Quad-SPI flash device on the Nexys4 DDR board

during manufacturing. The source code and prebuilt bitstream for this design are available for

download from the Digilent website. If the demo configuration is present in the flash and the Nexys4

DDR board is powered on in SPI mode, the demo project will allow basic hardware verification.

Here is an overview of how this demo drives the different onboard components:

• The user LEDs are illuminated when the corresponding user switch is placed in the on position.

• The tri-color LEDs are controlled by some of the user buttons. Pressing BTNL, BTNC, or BTNR causes them to illuminate either red, green, or blue, respectively. Pressing BTND causes them to begin cycling through many colors. Repeatedly pressing BTND will turn the two LEDs on or off.

• Pressing BTNU will trigger a 5 second recording from the onboard PDM microphone. This recording is then immediately played back on the mono audio out port. The status of the recording and playback is displayed on the user LEDs. The recording is saved in the DDR2 memory.

• The VGA port displays feedback from the onboard microphone, temperature sensors, accelerometer, RGB LEDs, and USB Mouse.

• Connecting a mouse to the USB-HID Mouse port will allow the pointer on the VGA display to be controlled. Only mice compatible with the Boot Mouse HID interface are supported.

• The seven-segment display will display a moving snake pattern.

All Nexys4 DDR boards are 100% tested during the manufacturing process. If any device on the

Nexys4 DDR board fails test or is not responding properly, it is likely that damage occurred during

transport or during use. Typical damage includes stressed solder joints and contaminants in switches

and buttons resulting in intermittent failures. Stressed solder joints can be repaired by reheating and

reflowing solder and contaminants can be cleaned with off-the-shelf electronics cleaning products. If

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a board fails test within the warranty period, it will be replaced at no cost. Contact Digilent for more

details.

Assigning ports to pins For the ports in the module (SWITCHES and LEDS) to control the components on the board, they

must be connected to pins on the FPGA. To do this, right click on the FPGA DEVICE (depending

on your board) icon and select New Source (Figure 2). In the popup window select to add an

Implementation Constraints File. Give it a name and click next. At the next screen click finish.

The ucf file is place under the top module entity. You can edit it by double clicking it (Figure 3).

Figure 2.Creating a new ucf file

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Figure 3. Editing the UCF File

Figure 4. Language Template Button: Display syntax of UCF commands

You can see the syntax for the different constraints that can go into the UCF file by clicking on the

language templates button and expanding the UCF folder (Figure 4). To see how to determine a

pin location constraint click the language templates button and then go to UCF → FPGA →

Placement → *BOARD_NAME *→ I/O. The syntax is as follows:

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NET "<port_name>" LOC=<pin_location>;

The pin locations for the switches and leds are given in Table I. Also refer to the online board user

guide for more details (Pages 13 and 17)

TIPS:

• You can add comment to a UCF using the # symbol.

• You need to make all the connections to the bus explicitly. For example for a bus signal with

two bits, bus[1:0] you should specify:

o NET bus[0] LOC=<pin_location>;

o NET bus[1] LOC=<pin_location>;

Generating a programming file

Programming the FPGA with impact Prerequisites

• A Digilent FPGA Board. • Vivado installed. See this tutorial to install Vivado.

o This guide uses version 2016.4, other versions can be used, but there may be differences.

• Digilent Board Files installed to handle selecting complicated configuration settings. See this tutorial.

Getting Started with Vivado

Prerequisites

Prior to starting this guide make sure to install Vivado. For more information see our Installing

Vivado guide.

Make sure to install Digilent's board files to make selection of a target board easier.

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Introduction

The goal of this guide is to familiarize the reader with the Vivado tools through the hello world of

hardware, blinking an LED.

This guide was created using Vivado 2016.4.

1. Starting Vivado

Windows

Open the start menu or desktop shortcut created during the installation process.

Linux

Open a terminal, cd into a working directory that can be cluttered with temporary Vivado files and

logs, then run the following: source <install_path>/Vivado/<version>/settings64.sh && vivado

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2. The Start Page

This is the screen that displays after Vivado starts up. The buttons are described below using the

image as a guide.

1. Create New Project

This button will open the New Project wizard. This wizard steps the user through creating a new

project. The wizard is stepped through in section 3.

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2. Open Project

This button will open a file browser. Navigate to the desired Xilinx Project (.xpr) file and click Open

to open the project in Vivado. 3. Open Example Project

This will guide the user through creating a new project based on an example project. These projects

will not work on all devices. 4. Open Hardware Manager

This will open the Hardware Manager without an associated project. If connecting to and

programming a device is all that is required by the user this is the button to use.

3. Creating a New Project

3.1

From the start page, select the Create New Project button to start the New Project Wizard.

3.2

The text in this dialog describes the steps that will be taken to create a project. Click Next to continue

to the first step.

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3.3

The first step is to set the name of the project. Vivado will use this name when generating its folder

structure.

Important

Do NOT use spaces in your project name or location path. This will cause problems with Vivado.

Instead use an underscore, a dash, or CamelCase.

Click Next to continue.

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3.4

Now that the project has a name and a place to save its files we need to select the type of project we

will be creating. Select RTL Project and make sure to check Do not specify sources at this time.

Source files will be added and created after the project has been created. Advanced users may use the

other options on this screen, but they will not be covered in this guide.

Click Next to continue.

3.5

Important

If your board does not appear in this list, then Digilent's board files haven't yet been installed. If this

is the case, revisit the prerequisites section of this guide, then close Vivado and start again from the

beginning.

Now it is time to choose the target device. Click the Boards tab at the top of the dialog, then select

your board from the list.

Click Next to continue.

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3.6

The next section gives a summary of the options selected throughout the wizard. Verify that the

information looks correct and click Finish.

4. The Flow Navigator

The Flow Navigator is the most important pane of the main Vivado window to know. It is how a user

navigates between different Vivado tools.

The Navigator is broken into seven sections:

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• Project Manager o Allows for quick access to project settings, adding sources, language templates, and

the IP catalog • IP Integrator

o Tools for creating Block Designs • Simulation

o Allows a developer to verify the output of their deisgn prior to programming their device

• RTL Analysis o lets the developer see how the tools are interpreting their code

• Synthesis o Gives access to Synthesis settings and post-synthesis reports

• Implementation o Gives access to Implementation settings and post-implementation reports

• Program and Debug o Access to settings for bitstream generation and the Hardware Manager

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5. The Project Manager

This tool is where most development will occur and is the initial tool open after creating a new

project.

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The Project Manager consists of four panes, Sources, Properties, Results, and the Workspace.

The Sources pane contains the project hierarchy and is used for opening up files. The folder structure

is organized such that the HDL files are kept under the Design Sources folder, constraints are kept

under the Constraints folder, and simulation files are kept under the Simulation Sources folder. Files

can be opened in the Workspace by double-clicking on the corresponding entry in the Sources pane.

Sources can also be added by either right clicking the folder to add the file to and selecting Add

Sources or by clicking the Add Sources button ( ).

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The Properties pane allows for viewing and editing of file properties. When a file is selected in the

Sources pane its properties are shown in here. This pane can usually be ignored.

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The unnamed pane at the bottom of the Project Manager window consists of several different useful

tools for debugging a project. The most important one to know is the Messages tool. This tool parses

the Tcl console for errors, warnings, and other important information and displays it in an

informative way.

These tools can be accessed by selecting the different tabs at the bottom of this pane.

The Tcl Console is a tool that allows for running commands directly without the use of the main user

interface. Some messages may link to the Tcl Console to provide more information regarding an

error.

The Reports tool is useful for quickly jumping to any one of the many reports that Vivado generates

on a design. These reports include power, timing, and utilization just to name a few.

The Log displays the output from the latest Synthesis, Implementation, and Simulation runs. Digging

into this is usually not necessary as the reports and messages view store the information in the log in

a more readable format.

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The last tool is the Design Runs. Using this tool run settings can be edited and new runs can be

created. This tool is useful when targeting multiple devices with the same design.

The most important pane in the Project Manager is the Workspace. The Workspace is where reports

are opened for viewing and HDL/constraints files are opened for editing. Initially the Workspace

displays the Project Summary which show some basic information from some of the reports.

6. Adding a Constraint File

In order to connect HDL code with the physical pins of the FPGA, a constraint file needs to be added

or created. Digilent has produced a Xilinx Design Constraint (XDC) file for each of our boards.

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Download the ZIP Archive containing each of these master XDC files, then extract it in a location

you will remember.

6.1

In the Project Manager section of the Flow Navigator, click the button. In the wizard

that pops up, select Add or create constraints then click Next.

6.2

At this stage, Vivado provides a list of all of the constraint files that will be added or created when

we click Finish. Currently this list is empty, this will change when files have been added or created.

A constraint file will not be created from scratch in this guide, so click Add Files.

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6.3

Find the directory you extracted the digilent-xdc-master.zip archive into, then click on the file for

your board. This should add the name of the file to the File Name field.

Click OK to continue.

6.4

Make sure that the selected XDC file has been added into the list of sources, then click Finish.

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6.5

In the Sources pane of the Project Manager, expand the Constraints folder, then double click on the

XDC file you just added. Each of Digilent's XDC files contains constraints for each of the commonly

used peripherals on their respective boards. For this demo, constraining the default system clock and

a single led is required.

Find and uncomment the lines that call get_ports on the names led[0] and clk by removing the '#'

symbol at the beginning of the line. On some boards the clock port will consist of two different ports,

clk_p and clk_n. The clock port is occasionally named something like sysclk, but should appear at

the top of the XDC file. Uncomment the create_clock line that follows the clock port/s definition as

well.

Tip

A board using clk_p/clk_n pins means that the input clock that uses differential logic. If you want to

know more read this article on low-voltage differential signalling.

Change the name inside of the get_ports call to 'led' from 'led[0]'. Do the same for the clock if it is

something other than 'clk' or 'clk_p' and 'clk_n'.

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7. Creating a Verilog Source File

7.1

In the Project Manager section of the Flow Navigator, click the button again. Select

Add or create design sources then click Next.

7.2

As before, at this stage, we will be provided a list of all of the source files that will be added or

created when we click Finish. Instead of clicking Add Files, click Create File.

Tip

It is also possible to add existing source files in the same way as we added the constraint file above.

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7.3

You will be prompted to select a File type, File name, and File location. Make sure to pick Verilog

and <Local to project> for the type and location. Give your file a name ending in '.v'.

Important

Do NOT use spaces in your file name. This will cause problems with Vivado. Instead use an

underscore, a dash, or CamelCase.

Click OK to continue.

7.4

Make sure that the new Verilog source file has been added into the list of sources, then click Finish.

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7.5

Unlike when the constraint file was added, at this point a Define Module dialog will pop up. You can

rename your Verilog module using the Module name field, but this is unnecessary. The Verilog

module's clock and led ports need to be defined. Clicking the Add ( ) button will add an empty slot

for a port to the I/O Port Definitions list.

There are five fields to define for each of the module's I/O ports:

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• Port Name: This field defines the name of the port and needs to match one of the names you used in your XDC file.

• Direction: This drop-down menu can be set to input, output, or inout, defining the direction that signals propagate through this port, with respect to your module. Outputs are the signals that your module will be controlling.

• Bus: This can be checked or not, when checked, this port consists of multiple single bit signals, grouped into a single bus.

• MSB: The index of the most significant bit of the port, if it is a bus. This option is grayed out for single-bit ports.

• LSB: The index of the least significant bit of the port, if it is a bus. This option is grayed out for single-bit ports.

Tip

If you are defining a module which will be instantiated in another module, which we will not go into

in this guide, be aware that the port names should not be declared in the XDC, this is only done for

your 'top' module.

If your board uses differential clocking, add two single-bit input ports with the same names as the

positive and negative clock ports that were uncommented in your XDC file. Otherwise, add a single

single-bit input port with the same name as the clock port that was uncommented in your XDC file.

Add a single-bit output port with the same name as the LED port that was uncommented in your

XDC file.

Once these two or three ports have been added, click OK to continue.

7.6

At this point, the new source file will be added to the Design Sources folder in the Sources pane of

the Project Manager. Expand this folder and double click on the file to open it.

Next, some Verilog code needs to be written to define how the design will actually behave.

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Between the ');' that comes after the module's port list and the 'endmodule' statement, add the

following code: reg [24:0] count = 0; assign led = count[24]; always @ (posedge(clk)) count <= count + 1; If your board is differentially clocked, add the following lines of code after ');' and before the 'reg

[24:0] count = 0;' line: wire clk; IBUFGDS clk_inst ( .O(clk), .I(clk_p), .IB(clk_n) ); It should be noted that the rate at which the clock will blink will differ depending on the board used.

System clocks on different Digilent boards run at a number of different rates, depending on the needs

of the board. The system clock period in nanoseconds can be found on the create_clock line of the

XDC file.

After completing this guide, it is suggested to try changing the provided Verilog code so that the

clock blinks at 1 Hertz - changing the XDC file beyond commenting or uncommenting entire lines is

not recommended.

8. Synthesis, Implementation, and Bitstream Generation

In order to create a file that can be used to program the target board, each stage of the “compilation

pipeline” needs to be run.

This starts with Synthesis. Synthesis turns HDL files into a transistor level description based on

timing and I/O constraints. To run Synthesis click either in the toolbar or in the

Flow Navigator. The output of Synthesis is then passed to Implementation.

Implementation has several steps. The steps that are always run are Opt Design (Optimize the design

to fit on the target FPGA), Place Design (Lay out the design in the target FPGA fabric), and Route

Design (Route signals through the fabric). To run Implementation click either in the toolbar or

in the Flow Navigator. This output is then passed on to the Bitstream Generator.

The Bitstream Generator generates the final outputs needed for programming the FPGA. To run

Bitstream Generation click either in the toolbar or in the Flow Navigator.

With no settings changed, the generator will create a '.bit' file.

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9. The Hardware Manager

The Hardware Manager is used for programming the target device.

The first step to programming a device is to connect the Vivado Hardware Server to it. There are two

ways to do this.

1. Open New Hardware Target

The first method is to manually open the target. This is required if the hardware is connected to

another computer. To get to the Open Hardware Target wizard either open the Hardware Manager

and click the link in the green banner or click the button in the Flow

Navigator under . From the drop-down that opens, select .

Once the wizard opens, click Next.

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The next screen asks if the hardware server is local or remote. If the board is connected to the host

computer choose local, if it is connected to another machine choose remote and fill in the Host Name

and Port fields.\

Click Next to continue.

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This screen gives a list of devices connected to the hardware server. If there is only one connected it

should be the only device shown. If there are multiple connected devices, determine the serial

number of the device to connect to and find it in the list.

Click Next to continue.

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The final screen shows a summary of the options selected in the wizard. Verify the information and

click Finish. The board is now connected to the hardware manager.

2. Auto-Connect

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The second method is to automatically open the target. To get to the button either

open the Hardware Manager and click the link in the green banner at the top of the

window or click the button in the Flow Navigator under . From

the drop-down that opens select . Vivado will attempt to find a hardware server

running on the local machine and will connect to the device on the server.

Programming

To program the device with the bit file generated earlier, either click the link in the

green banner at the top of the window or click the button in the Flow Navigator

under . From the drop-down that opens, select the device to program (Example:

) and the following window will open:

The Bitstream File field should be automatically filled in with the bit file generated earlier. If not,

click the button at the right end of the field and navigate to

<Project Directory>/<Project Name>.runs/impl_1/ and select the bit file (Example: ).

Now click Program. This will connect to the board, clear the current configuration, and program

using the new bit file.

10. Finished!

You should now see one of the LEDs on your board blinking!

Be sure to visit your board's resource center for more tutorials and demo projects. A link to each

resource center can be found here.

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Part 2: FPGA Verification of the 4-bit adder design In this design, you will utilize your 4-bit adder created from Assignment 1, and place it on the board

for functionality testing. The objective is to use the four switches as inputs to the adder, and the four

LEDs as outputs, so that the adder functionality is partially verified.

Adder Specifications The adder takes and adds two four-bit inputs, A and B, resulting into a four-bit output, C. You will

tie the bits of Input A onto the switches (MSB to switch 3, LSB to switch 0), and you will hardwire

the bits of Input B onto various preset input vectors using VHDL assignment statements. Use Xilinx

ISE to create your top level module which should take four bits as an external input (A), and

generate 4 bits as an external output (C). For testing purposes, we will take overflow into

consideration; however, you need to set your design to output 1111 if there is an overflow

rather than output a Carry-Out bit as we only have four LEDs.

Your B input needs to be coded into the VHDL module – every time you are to change the input, you

need to re-wire it and re-assign the new B values to the B input of the adder. Your A input will come

from the four switches. Tie the bits of A into each switch, and test your adder using this B sequence

of bits: a) 1101b) 1010c) 1100d) 1000e) 0000f) 0010g) 0101h) 0011 Your carry outputs will need to be tied to the LEDs on the board, with the MSB to the LED3 and the

LSB to the LED0 and so on.

Use the reference chart from Part 1 to determine the connections.

Synthesize and download your design(s) to the board and test the functionality of your adder. Make

sure that for every B combination given to you, you test the overflow case.

Part 3: 4-bit Counter - design and FPGA verification For the purposes of this assignment, you will design a 4 bit counter, which counts up, down and in a

Gray encoding fashion. The Gray code is listed below for your reference:

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4-bit Gray code

0000

0001

0011

0010

0110

0111

0101

0100

1100

1101

1111

1110

1010

1011

1001

1000

The counter will take as input two control signals, C1 and C2. When C1 is active, the counter counts

up (from 0000 to 1111). When C2 is active, the counter counts down (starting from 1111 down to

0000). The counter loops through the values until reset is pressed. When both C1 and C2 are active,

the counter counts the Gray code, starting from 0000 and looping until reset is pressed. At any

occasion, when the control signals change, the counter resets to its starting position and recounts.

The counter outputs the four-bits of its counting value.

For the purposes of testing this circuit, you will need to tie the control signals in your switches on the

FPGA Board. You will also tie the signals of the output bits of the 4-bit counter to the 4 LEDs on the

board.

In order however to be able to view the LEDs, you are to design a clock divider circuit, which takes

as input the system clock and generates a local clock for your counter, with a much lower frequency.

You are to use an internal counter inside the clock divider to measure the amount of cycles you are

going to generate the local clock to be sent to the external, top-level counter. You may have to play

with the frequency necessary to the human eye to make the LEDs switch visibly. A good value

would be to generate a 1-second clock cycles – hence the local clock should tick every 50 000 000

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system clock cycles for a 50MHz system clock). You can find more information about this to the

following link:

https://reference.digilentinc.com/learn/programmable-logic/tutorials/counter-and-clock-divider/start

Design the necessary components in VHDL in Xilinx ISE. Once your simulation shows that your

design is functioning correctly, use the steps indicated in Part 1 and download your design to the

board.

For documentation and links to online help, see the website. The links below (on website) may also

be useful.

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Turn-In Instructions Deliverables The ISE project folder containing the code and simulation files for all three parts and a lab report will be submitted electronically through email to me. Please use compression software to (winRar or 7zip) to keep the file size at a minimum. If the files are too large to be submitted via email please use the UCY file sending service https://filesender.ucy.ac.cy/filesender/.

• Please notify me in advance if you are going to delay delivering the lab

Also consider

• You are advised to look at the tutorials posted on the lab web site for a more comprehensive overview and detailed explanation of the tools.

• You are encourage to visit the Xilinx user community forums (http://forums.xilinx.com/). The community forum enables Xilinx FPGA users to share, discuss, and resolve issues related to the Xilinx tools, HDL, FPGA programming and more.

GOOD LUCK

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